The present invention relates to air data computers (ADCs) or systems, and algorithms implemented therein, for use in helicopters or other rotorcraft. More particularly, the present invention relates to a Low Airspeed Assist (LAA) method, and ADCs or systems implementing the same, to provide more accurate airspeed information in helicopters or other rotorcraft at low airspeeds.
Air data systems on helicopters and other rotorcraft include one or more pitot-static probes, an air data computer connected to the pitot-static probe(s), and other sensors if desired. The pitot-static probe(s) are mounted toward the front end of the helicopter, and are used to sense or calculate total pressure Pt and static pressure Ps If the aircraft utilizes more than one pitot-static probe, Pt is the average total pressure and Ps is the average static pressure measurement. As is known in the art, impact pressure (also referred to as dynamic pressure and commonly denoted Qc) can be calculated by subtracting the static pressure Ps from the total pressure Pt. These pressure measurements are most accurate when the pitot-static probe is properly placed on the aircraft to minimize aerodynamic errors from the vehicle, and when the pitot-static probe is sensing pitot and static pressure in a pressure field created only by the forward velocity of the vehicle and is free of the rotor downwash effect seen during slow flight, take-offs, and landings.
At low airspeeds, the impact pressure readings from the pitot-static probe(s) are affected by downwash from the main rotor into the static ports of the pitot-static probe(s), by high pressure fields beneath the rotor, and by the local pitch of the aircraft during takeoff. These effects cause the impact pressure reading to be less than the free stream impact pressure, thus resulting in negative values of impact pressure during such an event.
Conventionally, the impact pressure Qc is calculated as indicated above, and then that impact pressure is converted to an airspeed using known techniques. Using conventional techniques, with a negative impact pressure, calculated airspeed will be zero even though the helicopter in fact has a non-zero airspeed. As the helicopter increases in airspeed, relative to the helicopter, the downwash angles back toward the rear of the aircraft. At some minimum airspeed for the particular helicopter and flight conditions, the downwash moves completely behind the pitot-static probe(s), allowing accurate pressure measurements of impact pressure for use in the airspeed calculation.
During takeoffs and landings, it is important for estimated airspeeds to be readable and repeatable. Conventionally, measured helicopter airspeeds have only been accurate and repeatable when they have surpassed the minimum airspeed mentioned above, at which the downwash angles behind the pitot-static probe(s), relative to the helicopter. A minimum airspeed that is important to helicopter pilots is the Take Off Safety Speed (TOSS). The TOSS is the speed at which the aircraft will safely take to the air. This speed varies with aircraft, takeoff weight, air temperature, etc. An improved method of estimating helicopter airspeeds while the calculated impact pressure is still negative and the aircraft is accelerating through the Critical Decision Point (CDP—a point along the flight path that dictates the decision point on where to land, should landing be necessary) would be a significant improvement in the art.